Like an inductive element such as a coil, a resistive load generally has an inductance due to a presence of a wire inductance. When a transistor drives this type of load having an inductance, a counter-electromotive force occurs at the time of turn-off. In a load drive circuit, a switching power supply, an inverter circuit, etc., a freewheeling diode is connected in parallel with a transistor or a load to prevent occurrence of a counter-electromotive force. However, even in this case, since a surge voltage occurs with switching, a means of protecting a transistor from a surge voltage is needed.
The patent literature 1 has proposed a protection circuit where a Zener diode group is connected between the gate and drain of a MOSFET. When a surge voltage is applied to the drain, the Zener diode group breaks down. To reduce an increase in a gate voltage due to a breakdown, a Zener diode group is also connected between the gate and source of the MOSFET.